The highest speed IV a Pokemon with hidden power fire can have is 30. The max speed a Greninja can have with 30 speed IVs is 376 at level 100 or 190 at level 50, one off from maximum speed. According to my calculations, your Greninja has either 1-2 speed IVS (if the nature is Timid or a speed raising nature), or 31 if the nature is Modest (a nature that doesn't boost or lower it's speed), and given that 30 is the highest it can go for HP fire, I can safely assume that your Greninja has really low speed IVS.
I hope this helped you!
